Document HNI Internals

Document HNI internals, particularly gotchas:

e.g.,
- annotations are currently for optimizations only and can be removed
- can't always use invariant or if checks to make sure things like HHHBC can infer the types correctly. So you may have to remove a type hint on a property or something. e.g., a ?resource has to be removed when a mixed is assigned to it. 2958eab319d53ba15b0ddcb1cf3415af5f27aba7

Maybe also go through the step by step process of creating an HNI class from beginning to end.